Phd in CHEMICAL PROCESSES FOR INDUSTRY AND ENVIRONMENT

Thesis title: Valorisation of biogenic wastes via thermochemical processes for liquid biofuel production: a study on hydrothermal liquefaction

This doctoral research investigates the thermochemical conversion of biogenic wastes into liquid biofuel precursors, with particular emphasis on hydrothermal liquefaction (HTL) as a sustainable route for the up-cycle of waste biomass. The work addresses the challenge of processing high-moisture-content feedstocks, such as sewage sludge, while focusing on the development of fundamental understanding and practical strategies to produce high-quality biofuels from waste resources. The thesis is structured around three major parts, basing on their theoretical and experimental approaches: First, the work examines the hydrothermal liquefaction of model compounds (cellulose, albumin, and sunflower oil) to elucidate the reaction mechanisms controlling product yields and composition. A particular focus is directed toward the systematic investigation of heating rate effects on the HTL process, an underexplored parameter in existing literature. By varying the system heating rate while maintaining constant reactor scale, the study isolates the influence of heating rate on product distribution and qualitative composition, demonstrating that faster heating rates significantly improve biocrude yields and reduce the formation of solid residue. Complementary calorimetric measurements provide direct evidence of the thermal character of HTL reactions, enabling the development of a multiphase kinetic model based on the macromolecular composition of biomass. This model, adapted from the Hietala and Savage work, incorporates thermal balance considerations and predicts both product yields and reaction thermodynamics across various feedstocks. Second, the research applies HTL to the thermochemical conversion of real biogenic wastes, specifically multiple sewage sludge types (of municipal, paper mill, and agricultural origins). Comprehensive characterization and screening campaigns identified optimal process conditions for maximizing biocrude yield and energy recovery. The results demonstrate biocrude yields in the range of 20–30% on a dry basis with energy recoveries of 40–60% of the initial feedstock energy. However, the high protein content of sewage sludge results in biocrude with significant heteroatom content, particularly nitrogen (3–7% wt.), oxygen (~15% wt.), and minor sulphur fractions, which limits its direct applicability as a drop-in fuel. Batch scale comparison studies were conducted, to reveal the effects of thermal diffusion limitations on product distribution, emphasizing the critical role of heating dynamics in scaling up the technology. Third, the thesis explores innovative strategies to enhance the quality and commercial viability of liquid products. Hydrothermal upgrading (HTU) is investigated as a mild upgrading pathway that does not require external hydrogen supply, instead relying on in situ hydrogen generation through water splitting assisted by zero-valent metals (Fe and Zn) under hydrothermal conditions. Results demonstrate significant reductions in heteroatom content and improvements in H/C molar ratio when employing combined zero-valent metal and catalyst systems. The research also examines pyrolysis as an alternative thermochemical pathway for sewage sludge conversion and investigates co-pyrolysis strategies combining digested municipal sewage sludge with polystyrene waste to produce enhanced-quality products, suitable for subsequent co-processing integration into existing petroleum refining infrastructure. The overall findings demonstrate that HTL represents a viable and promising route for converting wet biogenic wastes, particularly sewage sludge, into energy-dense biocrude with acceptable energetic recovery. While direct use remains challenging due to heteroatom content, the integration of mild upgrading strategies such as hydrothermal upgrading provides a practical bridge toward commercial application. This work contributes to the scientific foundation necessary for the eventual industrial implementation of waste-to-biofuel processes, supporting the transition toward a circular bioeconomy aligned with sustainable development objectives.